1. Field of the Invention
This invention relates to a carrier for two-component electrophotographic developers and to a developer containing the carrier for use in copy machines, printers and the like.
2. Prior Art
Two-component developers used in electrophotography typically contain a toner and carrier. The carrier is such that it is mixed and agitated with the toner in a development box to impart a desired electrostatic charge to the toner particles and then carries the charged toner to static latent images on a photosensitive material to form corresponding toner images.
The carrier remains on a magnet and is recycled to the development box where the recycled carrier is again mixed and agitated with a fresh toner for repeated use.
Therefore, a carrier used in a developer is required as a matter of course to be unchanged and stable in characteristics and properties during its service period of time in order to enable the developer to maintain its desired image qualities (such as image density, fog, white spots or carrier scattering, gradation, and resolution) with minimal change and maximum stability not only at its initial stage of use but also during its entire period of use or service life.
In recent development system using a two-component developer, soft ferrites have been used as a carrier in place of conventional oxide-coated iron powder or resin-coated iron powder to obtain images of high quality. Typical of soft ferrites are MO.sub.a.M'O.sub.b (Fe.sub.2 O.sub.3)x wherein M and M' are each a metal element; and a, b and x are each an integer (The integer is a member like 1, 2, 3, 4 etc. A better way is to indicate x+a+b=1 (mol fraction)). Examples of the soft ferrites are Ni--Zn ferrite, Mn--Zn ferrite and Cu--Zn ferrite. These soft ferrite carriers have many of favorable properties for providing images of high quality as compared with iron powder carriers conventionally used; however, the use, in these carriers, of metals such as Ni, Cu and Zn has come to be avoided under rigorous environmental restrictions in recent years.
In view of environmental advantages, iron powder and magnetite powder carriers seem to be favorable. It is, however, difficult with these carriers to obtain an image quality and lifetime comparative to those obtained with the above mentioned soft ferrite carriers. From this standpoint, the ferrite carriers have been used widely, permitting their lifetime to be long as compared with the iron powder carrier. A further longer lifetime, however, has been desired.
From the viewpoint of environmental advantages, Li--Mn ferrites seem to be favorable among the ferrite carriers that have conventionally been proposed (Japanese Patent Application Laid-Open Gazette No. SHO 62-297857). Lithium, however, has not been used in practice because it is liable to be affected by its surroundings of, for example, temperature and humidity whereby it greatly varies in properties.